The applications of flexible heating elements are ubiquitous and includes such applications as heating pads, cup warmers, and food warmers. Flexible heating elements are typically made from a grid network of wire embedded in an electrically insulating fire resistance matrix that protects the end user from electrical shock and does not ignite when the heating element is used.
Various designs exist for forming heating elements in which a grid pattern is deposited on a flexible substrate. In one type of design electrically conductive patterns are deposited on a substrate from a liquid composition that is cured to produce the conductive pattern. However, these liquid compositions typically contain volatile organic compounds (“VOCs”) that are undesirable for a number of health and environmental reasons. For example, VOCs escape into the atmosphere resulting in water and air pollution. The cost of complying with strict government regulation on solvent emission levels is high. Moreover, the presence of VOCs in these liquid compositions severely limits the materials that can be used as substrates. For example, many types of polymeric substrates dissolve or are marred by VOCs. More importantly, many of the prior art liquid compositions do not produce conductive patterns that can withstand severe flexing that is desirable in flexible heating elements.
Generally, liquid compositions may be cured either by heating or by exposure to actinic radiation (i.e., UV light). Heat curable liquid in particular may be utilized to form electrically conductive coatings. However, such compositions almost always contain VOCs. Heat curable compositions also present other disadvantages such as slow cure times which lead to decreased productivity. Moreover, heat curable compositions require high energy for curing due to energy loss as well as the energy required to heat the coating. In addition to the limitations on substrate selection imposed by the VOCs, the need to heat cure limits substrates to materials that are heat tolerant at the curing temperatures.
Ultraviolet (“UV”) curable compositions may also be used to form electrically conductive coatings. Many UV curable compositions in the prior art also contain significant amounts of VOCs. Moreover, UV compositions tend to have high molecular weights and a substantial degree of cross linkage due to the highly reactive nature of the composition. As a result, many of these compositions suffer from low durability and resin shrinkage. With the use of many such compositions, an inordinately high amount of UV light is required to cure. New formulations that lessen these problems typically suffer from diminished abrasion, chemical, and scratch resistance as well as low thermal stability and adhesion. An additional disadvantage of typical UV compositions is their lack of stability which results in dispersion. With some compositions, suspended solids fall out of solution after a period of one to two days. Dispersion adversely affects the gloss and clarity of the finished product. To combat this problem, new compositions have been formulated with higher viscosities which often lessen the flowability of the composition. These viscous formulations rule out spray application and provide for an unsuitably high dipping thickness.
Accordingly, there exists a need for improved processes of forming heating elements on a substrate; and in particular for improved processes of forming heating elements on substrates that are flexible and/or damaged by VOCs.